Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
The kidneys, the supposed seat of the desires and affections; used metaphorically for "heart." The "reins" and the "heart" are often mentioned together, as denoting the whole moral constitution of man (Ps. 7:9, 16:7, 26:2, 139:13; Jer. 17:10, etc.).